The present invention relates in general to aerospace direct current (dc) power supplies and, more particularly, to an improved aerospace dc power supply wherein a neutral point controller including a full or partial controlling bridge and/or multiple wye and/or delta connected secondary circuits are used to convert alternating current (ac) power to dc power in an aerospace environment.
Early aircraft electrical systems operated on low voltage dc power, such as 6, 12 or 28 volts. Such systems permitted the use of commonly available batteries and accessories such as lighting fixtures to be used on aircraft. A generator was driven by an aircraft engine to charge the battery system. While some of these generators produced electrical noise, the battery system served as a large filter capacitance and attenuated the generator noise.
The advent of electrical instrumentation such as radios increased the amount of power required on the aircraft. Often, the added power demands of radios could be handled by the low voltage dc power systems in spite of the modest capabilities of distribution systems for low voltage dc power. However, when electric motors and other higher powered equipment began to appear on aircraft, the use of low voltage dc power and the required low voltage dc power distribution system became impractical. Even a modest amount of power such as 6 kilowatts (KW) required large, heavy, and inflexible cables to carry currents in excess of 200 amperes. Accordingly, ac aircraft power systems distributing power at 115 volts ac, 3 phase and 400 hertz (Hz) begin to emerge. The ac systems permitted power to be easily distributed and switched and the use of 400 Hz allowed electromagnetic devices such as motors and transformers to remain of modest size and weight.
Unfortunately, there is a problem in using an ac electrical system in that ac power is more difficult to produce and store than dc power. As a result, there has been a segregation of electrical equipment into loads that are imperative for the safety of flight of the aircraft from loads that are not essential. Safety of flight loads have remained as low voltage dc loads, such as the conventional 28 volt dc, which have access to power from a battery system should the engine mounted generators fail to operate.
While it is possible to have both ac and dc generators on an aircraft engine, the common practice is to use only ac generators. The use of only ac generators eliminates the need for heavy gauge feeder conductors in the wings and nacelles of an aircraft which would be required for high levels of dc power. Dc power, such as the conventional 28 volts dc, is converted from the ac power. The most common means for converting ac power into dc power is a transformer-rectifier (T-R) circuit.
The transformer receives ac power, such as three phase, 115 volts ac at 400 Hz, and drops the voltage to about 25 volts ac, line to line, at 400 Hz. The rectifiers then convert the low voltage ac into low voltage dc. Usually, such systems include filtering on both the input and output sides to attenuate most electromagnetic emissions conducted to the ac and dc electrical busses.
The simple transformer-rectifier has no active means for regulation of its output voltage which will vary in accordance with an applied load. The primary causes for the variations are the resistance voltage drops in the transformer windings, increases in voltage drop through the rectifiers with increased load, voltage drops in the ac feeders to the transformer-rectifier and voltage drops in the conductors carrying the power from the T-R circuit to the loads. In a typical aircraft power system the voltage from the transformer-rectifier may be 28 volts dc at 5 amperes of load, but may drop to 24 volts dc at 150 amperes of load.
A problem which arises with a system using a simple transformer-rectifier circuit is the loss of applied voltage to the battery. For example, a 24 volt battery system should be charged at about 28 volts dc to maintain proper charge and capacity. A system where the battery is maintained at 24 volts dc will lose capacity over time, and therefor reduce the amount of time the aircraft can continue to fly without engine generator power while the essential loads are powered by the battery system.
As an alternative, some development efforts have been applied to switch mode power supplies. Such power supplies are very complicated and of low reliability. And they tend to be expensive and inefficient converters of energy.
Another solution is to use a phase control bridge on the high voltage side of the transformer-rectifier circuit. This is preferable in systems where regulation is desired and cost is to be held down. The bridge is fully controlled using 6 thyristors or silicon controlled rectifiers (SCR's), arranged in three forward and reverse conducting pairs.
A neutral point controller is a simplified version where instead of using 6 controlled rectifiers arranged into conducting pairs, three controlled rectifiers are arranged in a ring, anode to cathode. This reduces the number of power control devices to three, no diodes or controlled rectifiers are needed to conduct in the reverse direction, less driver circuitry is required, reliability is improved, and waste heat rejection is lowered.
Frequently, this circuit includes a wye connected primary winding circuit which is magnetically coupled to both a wye connected secondary winding circuit and a delta connected secondary winding circuit. The wye and delta connected secondary winding circuits provide line to line voltages which are instantaneously displaced by 30.degree. electrical such that the voltages alternately peak to form twelve pulses per full power cycle. This results in a lower ripple output voltage.
Unfortunately, while twelve pulses per full power cycle are produced during substantially full conduction, as the conduction angle is reduced for smaller loads, the power output converges to three pulses per full power cycle hence raising the ripple output voltage. In addition, the delta connected secondary winding provides a path for circulating currents.
There is an ongoing need for improved aerospace dc power supplies which overcome the problems of the prior art and also which provide alternate configurations better adapted for specific applications. Such improvements not only serve to enrich the art but also to provide more efficient and reliable supplies for the aerospace industry.